High molecular weight, linear, saturated polyesters, as typified by polyesters of ethylene glycol and terephthalic acid, have found wide acceptance in the form of fibers (or filaments) and films as well as other configurations. They are, in general, colorless or clear, high melting, thermally and hydrolytically stable and are capable of being oriented, either mono- or biaxially, into extremely strong crystalline materials.
These polymers are, however, flammable and this places serious limitations upon their use in particular areas, for example in clothing (particularly children's clothing) in the form of fibers, and in electrical components, such as capacitors, in the form of films. Attempts have been made heretofore to improve the flame retardance of high molecular weight saturated polyester films, for instance by including halogen-containing additives therein. This has not been entirely satisfactory, however. Thus when such additives are mixed into the polymer batch before extrusion they may bloom out of the finished film over a period of time, or they may be removed (e.g. by contact with a porous solid or by solvent extraction). This has the effect of reducing the surface adhesion properties and transparency of the finished film or fiber. In addition, where the halogen-containing additive is removed, the flame retardance itself is lost.
Migration can be overcome or reduced by chemically binding flame-retardant agents in the polymer chain and, indeed, chemically-bound flame retardant agents, for example halogenated organic compounds, have been utilized in low to intermediate molecular weight unsaturated polyesters. Such polyesters have been of the amorphous (non-crystalline), relatively low melting, crosslinkable type which are generally suitable for use in the reinforced plastics and moulding arts but are unsatisfactory for use as film and fibers. Chemically bound flame retardants have not, however, been widely or successfully used in the high molecular weight linear saturated polyester area since they have been found to seriously and adversely affect the high melting points, high crystallinity, high thermal stability and high hydrolytic stability of these polymers (the combination of properties which make them valuable in the film and fiber areas).
As will be seen, the polymers of the present invention incorporate relatively large amounts of halogen (rendering them highly flame resistant) and at the same time retain the physical properties necessary for high quality films and fibers. It is this balance of properties which renders them especially valuable for use as flame resistant, high strength films and fibers.